title: Non-invasive measurements of cardiac output in atrial fibrillation: inert gas rebreathing and impedance cardiography.
abstract: BACKGROUND
  Atrial fibrillation (AF) is associated with significant morbidity and mortality. To test the effect of interventions, knowledge of cardiac output (CO) is important. However, the irregular heart rate might cause some methods for determination of CO to have inherent weaknesses. Objective. To assess the validity of these methods in AF, a new inert gas rebreathing device and impedance cardiography was tested with echocardiography as reference.
  
  
  METHODS
  Using a cross-sectional design, 127 patients with AF and 24 in SR were consecutively recruited. Resting CO was measured using inert gas rebreathing (n = 62) or impedance measurement of intrathoracic blood flow (n = 89) in separate studies with echocardiographic measurement as reference.
  
  
  RESULTS
  CO determined with impedance cardiography was mean 4.77 L/min ± 2.24(SD) compared to 4.93 L/min ± 1.17 by echocardiography (n = 89, n.s.) in patients with AF. CO by inert gas rebreathing was 4.98 L/min ± 2.49(SD) compared to 5.70 L/min ± 2.49 by echocardiography (n = 62, n.s.) in patients with AF and SR (AF 5.42 ± 2.9 vs. 6.27, n.s. and SR 4.09 ± 1.08 vs. 4.35 ± 0.86, n.s.). Mean bias between impedance cardiography and echocardiography was 0.14 ± 0.95 L/min and -0.13 ± 0.98 L/min between inert gas rebreathing and echocardiography. Inert gas rebreathing showed larger intra-patient variation than impedance cardiography (0.11 vs. 0.054). Correlation between inert gas rebreathing and echocardiography was r = -0.060 and between impedance cardiography and echocardiography was r = 0.128. Impedance cardiography and inert gas rebreathing both underestimated CO compared to echocardiography.
  
  
  CONCLUSIONS
  Variation between the inert gas rebreathing and the reference method for AF patients was less than desired. Impedance cardiography was superior to inert gas rebreathing and showed acceptable agreement with echocardiography and variability similar to echocardiography.
authors.affiliation: Department of Cardiology, Hvidovre Hospital, Faculty of Health Sciences, University of Copenhagen, Copenhagen, Denmark. philiposbak@yahoo.dk
authors.forename: Philip S
authors.initials: PS
authors.lastname: Osbak
authors.forename: Jens H
authors.initials: JH
authors.lastname: Henriksen
authors.forename: Klaus F
authors.initials: KF
authors.lastname: Kofoed
authors.forename: Gorm B
authors.initials: GB
authors.lastname: Jensen
mesh_terms: D000368:Aged; D000465:Algorithms; D001281:Atrial Fibrillation / Q000175:diagnosis / Q000503:physiopathology*; D001944:Breath Tests / Q000295:instrumentation; D002248:Carbon Monoxide / Q000032:analysis*; D002302:Cardiac Output; D002307:Cardiography, Impedance; D003430:Cross-Sectional Studies; D004452:Echocardiography; D005260:Female; D006334:Heart Function Tests; D006801:Humans; D008297:Male; D008875:Middle Aged; D009609:Nitrous Oxide; D010100:Oxygen; D013459:Sulfur Hexafluoride
chemical_list: D002248:Carbon Monoxide; D009609:Nitrous Oxide; D010100:Oxygen; D013459:Sulfur Hexafluoride